1. Field of the Invention
The present invention relates generally to pressure transducers, and more particularly to a high-pressure/high-temperature pressure transducer that senses diaphragm deflection for pressure sensing.
2. Description of the Related Art
A typical pressure transducer includes a pressure sensor that can constitute, e.g., a piezo-sensor that uses the piezo-resistive effect or a thin-film pressure sensor having thin-film measurement strips. The pressure sensor is supplied using electrical auxiliary energy in that an electrical voltage is applied to it and it is subjected directly or indirectly to the pressure of a measured medium. The electrical output signal from the pressure sensor changes depending on the effective pressure, the signal being forwarded to a circuit arrangement that, e.g., comprises a bridge circuit and/or adjustable resistances for null measurement, temperature compensation and setting nominal sensitivity. The pressure transducer provides an electrical output signal in the form of an output signal voltage or output signal current that is a measure of the pressure measured.
With pressure sensors with continuing electrical excitation, a portion of the electrical excitation is diverted into a signal channel by some distortion caused by the measured pressure. These may include variable capacitors, variable inductors (including variable transformers), and variable resistors. For reasons of simplicity and ruggedness, variable resistors are most commonly used. Resistive pressure sensors, generally, compare the resistance of a resistor modified by the action of the pressure to the resistance of a similar resistor either unmodified, or modified in the opposite direction by the action of the pressure. The comparison is generally made in a Wheatstone bridge. Strain gages are resistors designed to be changed by strain. Metallic strain gages may be very stable and have very low noise, but will produce a resistance change of only one or two parts per thousand as a practical full scale signal. Semiconductor strain gages may have resistance changes one hundred times that of a metallic strain gage. The greater signal makes their use desirable in environments that are noisy and/or remote from where the signal is used.
Pressure measurement in high temperature environments presents a challenge for the use of semiconductor strain gages (piezoresistors). Most commonly, the medium in which the pressure is to be measured is chemically aggressive. The portion of the transducer touching the medium must be resistant to any change the medium might cause. Almost always, the portion of the transducer touching the medium is a corrosion resistant metal. Metals generally, and particularly corrosion resistant metals, have thermal expansion coefficients in the range of ten to twenty parts per million per degree C. (Special low-coefficient metals are very susceptible to corrosion.) Common semiconductor materials have much lower coefficients of expansion, in the range of two to four parts per million per degree C. Silicon in particular expands about 3.3 ppm/degree C. The difference in expansion over temperature ranges of interest is potentially destructive to semiconductor strain gages, and may impose large undesired signals on them.
There is a need for a corrosion-resistant metallic pressure transducer that incorporates a piezoresistive measuring system.